Micropropagation is the process of growing new generation plants from a single piece of tissue that has been excised from a carefully selected parent plant or cultivar This process permits the mass reproduction of plants having certain desirable traits since substantially all of the new generation plants produced are genetically identical to and have all the desirable traits of the parent. The factors considered in selection of parent stock for micropropagation include vigor, growing habits, resistance to disease and insect attack; water, nutrient, temperature, and light requirements. In horticulture, perhaps most important is the quality of the bloom if it is a flowering plant. In agriculture it is more likely that growth rate and yield would be of principal concern.
Tissue culturing is the process of growing cells in vitro and is used to grow both plant and animal cells. Tissue culturing techniques are commonly used in the early stages of the plant micropropagation process where it is desirable to rapidly produce plant cells. The concept of tissue culturing is not new and has its early origins in 1902 from attempts to cultivate cells from leaves. See G. Haverlandt, "Sitz-Ber. Mat.-Nat. KI. Kais Akad Wiss. Wien III," 69 (1902). Although Haverlandt failed to obtain viable cultures, his work and theory served as a foundation for later attempts by other scientists. The first successful culture of cells was obtained as early as 1934. See R. J. Gautheret, "C.R. Acad. Sci." 198, 2195 (1934).
Plant propagation within the horticulture industry is carried out by one of two general techniques, namely seed propagation or vegetative propagation. Seed propagation is the most common method of plant propagation.
Seed propagation in the past has offered the grower the fastest method of producing plant material. In most plant varieties, large numbers of seed are produced annually, the seeds being easily collected and sown to produce the next generation of plants. Drawbacks to this technique exist. These include yearly fluctuations in seed production, exacting germination requirements, high seedling mortality, and lack of crop uniformity. Lack of uniformity is one of the most significant problems. Seeds are produced by plants according to the genetic variances governed by Mendelian rules. Basically each seed is genetically different and each will grow with its own specific traits.
Vegetative propagation includes three distinct methods: propagation from cuttings, by the division of existing plants, and through micropropagation. Vegetative propagation has offered the grower a degree of crop uniformity that is unattainable through seed propagation, as substantially all the plants produced using any of the three vegetative propagation methods are genetically identical to the parent plant. Thus, vegetative propagation has advantages over seed propagation since plants having certain desirable characteristics can be consistently reproduced. Further, vegetative propagation methods allow for the production of plants that produce little or no viable seed.
There are major drawbacks to the methods of propagation from cuttings and by the division of existing plants. These include the labor intensive operation necessary to produce the new plants and the time to develop a sufficient parent or stock reserve from which to obtain plant material, since each of these two methods requires the removal of relatively large portions of the plant. By contrast, micropropagation eliminates the problems associated with these two forms of vegetative propagation since in micropropagation only very small pieces of tissue are removed from the stock or parent plants. Thus, micropropagation retains the advantages common to all types of vegetative propagation, i.e., identity of progeny and the ability to propagate non-seed producing plants, while having the additional advantage that only a small piece of tissue from the cultivar or parent plant is required.
Despite these advantages over other methods of plant propagation, micropropagation is more commonly used to propagate horticultural strains than agricultural strains because micropropagation of agricultural plants is more time-consuming and costly than seed propagation. Further, plant micropropagation must be carried out under sterile conditions, i.e., in sterile laboratories and culture rooms rather than in the field or greenhouse. Horticultural plants lend themselves to this less efficient method because the plants need not be produced in the massive quantities associated with agriculture, and further, because the consumers--who are purchasing and growing plants as an avocation--are more willing to absorb the higher costs. Nevertheless, micropropagation is increasingly used in agriculture, particularly in the development of new crops.
Although horticulture is primarily an avocation, it is nonetheless important commercially. In fact, gardening is the most popular leisure-time activity, with active participants included in approximately 44% of all households in the United States. See "The National Gardener," Jan. 1987. Excluding food crops and fruit trees, total sales of green plants in the United States in 1985 approximated 25-30 billion dollars.
Improvements in tissue culturing techniques also have applications beyond the micropropagation of plants. Essentially the same culturing process is used to culture animal and even human tissue, such tissue being used in the fields of animal agriculture and human and veterinary medicine.
Micropropagation begins with the selection of a cultivar or parent plant. A cultivar may be selected from existing strains or may be created or developed through such methods as selected breeding and genetic engineering. Researchers are hybridizing plants having individually advantageous traits so as to produce a plant with predetermined characteristics. Further, with the advent of biotechnology, the biologist can transfer desirable genetic material from one plant to another, thus engineering a new plant (genetic engineering). Other genetic manipulations such as protoplast fusion are also being used.
Once the parent plant is selected or achieved through the use of one of the methods described above, micropropagation is used to reproduce the carefully selected or developed plant by means of tissue culturing and cell manipulation.
Great strides are now being made almost daily through genetic engineering and micropropagation via tissue culture. Disease-free clones, germ plasm, accelerated asexual propagation and in vitro propagation are becoming increasingly common in horticulture. Micropagated plants include woody ornamentals, tropicals, and blooming plants. The micropropagation process is a powerful force in the propagation of plant material. It allows mass production of quality plant material in a short period of time and offers definite advantages to both the grower and consumer.
The values of micropropagating plants include:
(1) a means of developing disease-free progeny;
(2) a method of asexual propagation that provides rapid multiplication of selected cultivars;
(3) the preservation of selected varieties (germ plasm); and
(4) the development of potentially valuable "sports", naturally occurring mutations.